Currently, many consumer electronics are powered by lithium-ion batteries, the safety of which is a big concern to both the consumers and the manufacturers. A reliable battery must survive several abuse conditions, including overcharge. Overcharge generally occurs when a current is forced through a lithium-ion battery and the charge delivered exceeds the charge-storing capability of the battery. Overcharge of lithium-ion batteries can trigger chemical and electrochemical reactions of battery components, rapid temperature elevation, and can even trigger self-accelerating reactions leading up to, and including, explosion of the battery.
In current lithium-ion battery technology, several overcharge protection mechanisms are typically added to ensure the safety of the batteries during overcharging conditions. For instance, a chemical compound known as a redox shuttle additive may be incorporated into the battery electrolyte to provide intrinsic overcharge protection. Generally, the redox shuttle can be reversibly electrochemically oxidized and reduced at a potential slightly higher than the working potential of the positive electrode of the battery. With the incorporation of a redox shuttle into the electrolyte, lithium-ion batteries can normally operate in a voltage range below the redox potential of the redox shuttle. If the battery is overcharged, the battery voltage will meet the redox potential of the additive first and activate the redox mechanism of the redox shuttle. In general, when the overcharge current is lower than the shuttle capability of the additive, the redox shuttle will be the only active component to transfer the excessive charge through the battery without causing any damage to the battery. Under such mechanisms, the dangerous voltage of the battery is never reached even if the battery is overcharged.
While redox shuttles provide some protection of a lithium-ion battery, no redox shuttle can provide unlimited overcharge protection. The main barrier is the maximum shuttle current the redox shuttle can provide, which determines the maximum overcharge current that a battery with a redox shuttle, can sustain. The maximum shuttle current is physically limited by the solubility of the redox shuttle in non-aqueous electrolytes, the diffusion coefficient of the redox shuttle in the non-aqueous electrolytes, the charge transfer constant of the redox shuttle on the electrode surface, and battery geometry. Generally, redox shuttles described in the literature have very limited solubility in the non-aqueous electrolytes, and can only provide low rate overcharge protection. Once the overcharge current exceeds the maximum shuttle current of the redox shuttle, the battery will be driven to higher voltages that trigger dangerous reactions in the battery.
Recently, lithium-ion batteries have been proposed as the power source for hybrid electric vehicles (HEV). During braking of a HEV, the excessive energy from the engine is stored in the lithium-ion battery. A high-rate pulse current, which can be up to a 10 C rate, will be forced through the battery to meet the high power output of the engine. In this situation, those lithium-ion cells already at their maximum charge capacity will be overcharged with a very high current (up to 10 C). It remains a huge challenge to design a redox shuttle to provide such high shuttle current. Because state-of-the-art redox shuttles alone cannot provide such high rate continuous overcharge protection, there remains a need in the art to meet the challenge of high rate pulse overcharge can for lithium-ion batteries.